The invention permits NOx reduction using urea instead of ammonia, even at low loads. The invention provides a new selective catalytic reduction of NOx, which is enabled by injecting urea into a bypass for a heat exchanger section, e.g., an economizer, of a boiler in advance of an SCR unit to enable sufficiently high temperatures for urea gasification under all load conditions. The flow through this bypass is adjusted to provide sufficient temperature and energy to decompose the urea into its active components including ammonia and to provide sufficient mass to mix with the bulk of the flue gas. Under high load conditions, the bypass can be almost fully closed and the affected heat exchanger can be operated normally without excessively cooling the combustion gases, using only a portion of bypassed gases which are hot enough to decompose the urea into its active components including ammonia.
Efforts are being made in many jurisdictions to reduce the emissions of nitrogen oxides (NOx). The technologies have included those that modify the combustion conditions and fuels, known as primary measures, and those that treat the exhaust after combustion, known as secondary measures. When effective primary measures are employed, the secondary measures can still be employed to achieve further reductions. To provide the best NOx reduction, it is apparent that both primary and secondary measures will be necessary.
Among the known secondary measures are selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR). Both have been conducted with both ammonia and urea. See, for example U.S. Pat. No. 3,900,554, wherein Lyon discloses SNCR of nitrogen monoxide (NO) in a combustion effluent by injecting ammonia, specified ammonia precursors or their aqueous solutions into the effluent for mixing with the nitrogen monoxide at a temperature within the range of 1600° F. to 2000° F. Lyon also suggests the use of reducing agents, such as hydrogen or various hydrocarbons, to permit the effective use of ammonia at effluent temperatures as low as 1300° F. However, these temperatures are often too high for effective treatment, ammonia is difficult to deal with safely, and SNCR is not as effective as SCR. Similar processes are taught for urea by Arand, Muzio, and Sotter, in U.S. Pat. No. 4,208,386, and Arand, Muzio, and Teixeira, in U.S. Pat. No. 4,325,924. Again the temperatures are high, and the use of lower temperatures is not enabled.
SCR can operate with ammonia at lower temperatures, generally within the range of from 100° to 900° F. In this regard, see U.S. Pat. Nos. 3,032,387 and 3,599,427. SCR (selective catalytic reduction) has been available for years in some contexts for reducing NOx. To date, however, SCR has depended mostly on the use of ammonia. Urea is safer, but has not been practical for many SCR applications due to the difficulty in converting it from a solid or an aqueous form to its active gaseous species that are reactive on catalyst bed for NOx reduction. Also, the reagent economics typically favor anhydrous ammonia over urea. In “A Selective Catalytic Reduction Of NOx from Diesel Engines Using Injection Of Urea” (Ph.D. thesis, September 1995), Hultermans describes a number of technical challenges in the context of Diesel engines while giving a broad background on the technology. Under low load conditions, the combustion gases are often cooled to temperatures so low that an aqueous solution of urea cannot be fully vaporized with the release of its active gaseous species.
The use of catalysts for NOx reduction utilizing urea is effective but is sensitive to particulates and undecomposed urea, which can foul a catalyst. In this regard, it must be remembered that temperatures at the low end of the SCR treatment temperature range will not be high enough to fully gasify the urea. In addition, SCR requires very uniform mixing of active gaseous species prior to contact with the catalyst, and it is difficult to uniformly mix urea or its decomposition products with the large amounts of effluent in need of treatment. The limited attempts to use urea SCR for stationary and mobile sources, such as diesel engines, have been described in several recent patents including U.S. Pat. No. 5,431,893, to Hug, et al. To protect the catalyst from fouling, Hug, et al., proposes bulky equipment capable of treating all effluent with urea. Regardless of physical form, urea takes time to break down in hot exhaust gases and may cause nozzle plugging at the temperatures most conducive to gasification. This disclosure highlights the problems making it a necessity that the urea solution is maintained at a temperature below 100° C. to prevent hydrolysis in the injection equipment, and they also use dilute solutions to avoid hydrolysis in the equipment and this adds to the need for heat to evaporate the water. See also, WO 97/01387 and European Patent Specification 487,886 A1.
In European Patent Specification 615,777 A1, there is described an apparatus that feeds solid urea into a channel containing exhaust gases, which are said to be hydrolyzed in the presence of a catalyst. For successful operation the disclosure indicates that it is necessary to employ a hydrolysis catalyst, compressed air for dispersion of fine solids, means for grinding the urea into fine solids and a coating to prevent urea prills from sticking together. Despite achieving the goal of removing water from the process, the specification introduces solid urea into the gas stream—possibly depositing urea on the SCR catalyst.
In U.S. Pat. No. 6,146,605 to Spokoyny, there is described a combined SCR/SNCR process in a staged process involving a separate step of hydrolyzing the urea prior to an SCR stage. A similar process is disclosed in U.S. Pat. Nos. 5,985,224 and 6,093,380 to Lagana, et al., which describe a method and apparatus involving the hydrolysis of urea followed by a separation of a gas phase from a liquid hydrolysate phase. Also, Copper, et al., disclosed a urea hydrolysis process to generate ammonia in U.S. Pat. No. 6,077,491. In all of these processes there is a requirement to handle a significant amount of high temperature and high pressure gas and liquid phases containing ammonia during and after hydrolysis. This extra processing requires the purchase and maintenance of additional equipment and emergency plan and equipment to handle ammonia release in case of process failures, and it would be desirable to have a system which operated more safely, simply and efficiently.
EP 0 363 684 mentions urea as an alternative to ammonia and doesn't change the processing to accommodate it. The reference describes a large evaporation tank to vaporize ammonia. It has as its main purpose the mixing of steam and ammonia. It would be desirable to enable the use of urea instead of ammonia even at low load conditions. And, EP 1 052 009 utilizes urea for SCR but requires a relatively large volume of gas and a urea hydrolysis catalyst for breaking down urea to NH3 and CO2. It would be desirable to utilize urea without need for a separate reactor containing a urea hydrolysis catalyst.
The art is awaiting the development of a process and apparatus that would permit the use of urea in an SCR process simply, reliably, economically, and safely, even under conditions of low load.